The present invention relates generally to parallel multiple lamp ballasts. More particularly, the invention relates to protecting parallel multiple lamp ballasts from unsafe high voltage conditions following removal of a lamp from connection with the ballast and protecting parallel multiple lamp ballasts from unbalanced loads which can affect reliability of the ballast.
Parallel multiple lamp ballasts are widely used due to their low cost and low load impedance, especially when operating at dimmed output levels. Sensing removal of a lamp or of unbalanced lamp currents is difficult and expensive because the lamps are connected in parallel with respect to an oscillator (e.g., inverter) and resonant tank of the ballast. This difficulty is amplified when the ballast is operating at a dimmed output level. If a lamp is removed from electrical connection with the ballast during operation of the ballast, the voltage at the contacts that the lamp was removed from will become excessive and unsafe if the ballast continues operating normally. Further, unbalanced loads can cause breakdown in various components of the ballast if the unbalanced load becomes too great (e.g., when a lamp is removed from connection with the ballast while the ballast is operating). This can reduce the reliability of parallel multiple lamp ballasts.